An electro-photographic image forming apparatus provided with a heater for thermally fixing developer images is well known in the art. To thermally fix the developer image, the heater heats a sheet of paper onto which a developer image has been transferred. Since large power is required for the heater to operate, large inrush current abruptly flows into the heater from a power source connected to the image forming apparatus when the heater is started up. This large inrush current may cause a power source voltage (i.e., a voltage level of the power source) to be lowered or dropped. This voltage dropping of the power source voltage adversely exerts influence not only upon operation of the image forming apparatus but also upon operations of other apparatuses connected to the same power source.
Japanese Patent Application Publication No. H11-202680 discloses an image forming apparatus provided with a thermally fixing heater. The disclosed apparatus performs a phase control when starting up the heater, thereby restraining a power source voltage from being abruptly dropped.